Automatic computer control of kiln systems has been a challenge for control engineers in the cement industry ever since the appearance of process computer systems in the mid-sixties. The first attempts were based on the application of multivariable control theory together with mathematical models of the kiln systems. The results were rather disappointing and the systems never matured to a state, where they became commercially available.
Starting in the mid-seventies a different approach to computer control of cement kilns was pioneered and became known as the FLS-SDR/Fuzzy computer control system. This is described in a publication FLS-review 67, which is a reprint of an article entitled "Control of a cement kiln by fuzzy logic" by L. P. Holmblad and J.-J. Ostergaard, published by North-Holland Publishing Company, 1982. Briefly, based on the concept of Fuzzy Logic the control strategy was formulated as a set of linquistic control rules transferring the experience of human kiln operators to an automatic computer control program. The FLS-SDR/Fuzzy computer control system has been marketed since 1980 and has today been installed on a considerable number of kilns. These systems have clearly demonstrated that the Expert System approach embedded in the Fuzzy control concept is capable of efficiently controlling different kiln systems, and on-line control figures of 85-90% are usually achievable.
However, up until now automatic control has been applied in more or less stead-state operation only after manual start-up of the kiln. The controller usually takes over when production has reached 70-100% of normal load, depending on the duration of the kiln stop; the shorter the stop, the earlier the controller can usually be switched on.